1. Field of the Invention
The present invention relates to the process of transmitting data across a computer network. More specifically, the present invention relates to a method and an apparatus for offloading the task of message segmentation from a central processing unit to a network interface card
2. Related Art
Modern communication protocols allow applications executing on different computer systems to communicate with each other. Typically, an application desiring to communicate with another application on another computer system passes the message to a communication process associated with a protocol stack. This protocol stack provides many different mechanisms to transport messages between applications. For example, the Transmission Control Protocol/Internet Protocol (TCP/IP) stack provides reliable delivery of messages from one application to another.
Typically, the TCP layer on the sending end segments a message into maximum segment size (mss) units and transmits the segments to the TCP layer on the receiving end. The TCP layer on the receiving end ensures that each segment is reliably received and that the segments are in proper order.
The IP layer, which operates under the TCP layer, passes these segments from the TCP layer on the sending end to a counterpart IP layer on the receiving end. The IP layer supports addressing that allows segments to be delivered to the proper receiver. However, the IP layer is not reliable. Segments may be lost in transit over the physical network, or the segments may be delivered out of order.
A physical layer resides under the IP layer and typically includes an Ethernet interface. The Ethernet interface facilitates addressing for the sending and receiving ends of the physical link. In doing so, an address resolution protocol (ARP) associates the IP address to a corresponding Ethernet address.
Note that the maximum transmission unit (mtu) for the physical layer limits the mss. The mtu for Ethernet, for example, is 1500 octets (eight-bit bytes). Unfortunately, the relatively small mss allowed by this mtu can cause a performance bottleneck for high-bandwidth communications over modem high-speed networks. This is because performing the segmentation at the central processing unit (CPU) of a computer system causes the CPU to spend considerable time segmenting the messages, which can significantly limit the availability of the CPU to perform mainstream processing.
What is needed is a method and an apparatus for offloading the segmentation process from the CPU, thereby freeing the CPU to do productive work.